An engine may be operated with one or more dedicated EGR cylinders (e.g., a cylinder that directs at least a portion of its exhaust flow, without exhaust from other cylinders, to provide external EGR to engine cylinders) that direct all of their exhaust gas to the intake air of engine cylinders as external exhaust gas recirculation (EGR). This arrangement may allow the engine to operate with higher levels of exhaust gas dilution. Consequently, engine pumping work may be reduced and engine efficiency may be improved. The external EGR may also be directed through a cooler to reduce gas temperatures in the engine's cylinders, thereby making the engine less knock limited and further reducing NOx emissions. However, condensation may form within the EGR cooler, and the condensation may eventually be drawn into the engine where it may increase the possibility of engine misfire.
The inventors herein have recognized the above-mentioned disadvantages of operating a highly diluted engine and have developed a method for an engine, comprising: recirculating exhaust gas from only a dedicated EGR cylinder to an intake of engine cylinders; increasing exhaust gas temperature in the dedicated EGR cylinder via increasing an air-fuel ratio of the dedicated EGR cylinder in response to EGR cooler condensation; and operating remaining engine cylinders about a stoichiometric air-fuel ratio in response to EGR cooler condensation.
By increasing an air-fuel ratio of a cylinder that is operating rich to improve engine combustion stability of an engine that includes a dedicated EGR (DEGR) cylinder, it may be possible to increase EGR gas temperature and removed condensation from an EGR cooler. A DEGR cylinder may typically be operated at a rich air-fuel ratio to improve combustion stability in an engine's non-DEGR cylinders. However, if higher EGR gas temperature is desired based on EGR cooler condensation, a cool engine, and/or low engine speed and load, EGR gas temperature may be increased via increasing or making less rich an air-fuel mixture supplied to a DEGR cylinder. In addition, when the air-fuel ratio of the DEGR cylinder is increased, resulting in less fuel in the recirculated exhaust gas, the amount of fuel injected to each of the remaining cylinders may be increased to maintain a stoichiometric air-fuel ratio in the remaining cylinders. Additionally, in some examples, spark timing of the DEGR cylinder may be retarded as compared with spark timing of non-DEGR cylinders. In this way, EGR gas temperature may be increased without increasing exhaust temperature of non-DEGR cylinders.
The present description may provide several advantages. In particular, the approach may reduce the possibility of engine misfire by allowing EGR cooler condensation to be reduced. Further, the approach may provide faster engine warm-up, thereby reducing engine emissions. Further, the approach may improve engine efficiency at light engine loads.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.